A terahertz wave measuring apparatus is an apparatus that irradiates a measured object with a terahertz wave and observes a reflected wave or a transmitted wave to detect an intensity change or a phase difference generated by the measured object. A terahertz wave measuring apparatus is an apparatus that is effective for non-destructively observing components contained in the measured object.
The terahertz wave is an electromagnetic wave in a frequency range of substantially 0.1 THz to 10 THz and has features superior in transmissibility to many substances such as paper, wood, and plastic compared with a far infrared ray that is the electromagnetic wave of a higher frequency band, and superior in rectilinearity and resolution compared with a millimeter wave that is an electromagnetic wave of a lower frequency band.
In addition, an intrinsic spectrum of many substances including polymer compounds such as sugars and proteins is included in the frequency band of the terahertz wave. It is possible to non-destructively observe a shape and an internal structure of the measured object, presence/absence of defects/foreign matters, a difference in material and contained component, and the like while being in a transparent container by taking advantage of these features, by irradiating the measured object with the terahertz wave, and by observing the transmitted wave or the reflected wave. In addition, it is possible to non-destructively observe surface irregularities of the measured object, a thickness of a layer in a layer structure, and an internal structure such as a hollow by detecting a difference in phase between a terahertz wave reflected on the measured object and a terahertz wave reflected on a reference metal. Therefore, it is expected that a wide range of terahertz wave application technologies applicable to pharmaceutical inspection, material inspection, and structural inspection will be realized in the future.
However, since the terahertz wave technology of the related art is difficult to achieve both a highly output terahertz wave generator and highly sensitive terahertz wave detector, and it is difficult to be downsized, the spread of terahertz wave technology is not progressed for industrial applications such as a product inspection apparatus. Therefore, research and development of technologies that can achieve downsizing, high output and high sensitivity are promoted.
As a terahertz wave measuring system recently developed in order to achieve the high output and high sensitivity of the terahertz wave, for example, an injection-seeded Terahertz-wave Parametric Generator (is-TPG) disclosed in NPL 1 is known. This technique includes a method for generating the terahertz wave having high intensity by improving a method for generating a terahertz wave described in PTL 2 and a detecting method with high sensitivity using a nonlinear optical crystal.
A feature of this technique is that the nonlinear optical crystal is used both for the terahertz wave for generation and the terahertz wave for detection. When the nonlinear optical crystal for generation is irradiated with a pump beam and a seed beam having two wavelengths at an appropriate angle, a narrow-band terahertz wave is generated as a difference frequency component. In addition, when the nonlinear optical crystal for detection is irradiated with the pump beam and the terahertz wave at an appropriate angle, the infrared light having the same wavelength as that of the seed beam is outputted as a detection light. It is possible to utilize various infrared sensors that are highly sensitive to the observation of a detection light, to observe the terahertz wave which is weaker than that of a system of the related art using a silicon bolometer or pyrometer that directly observes the terahertz wave, and to obtain a high SN ratio of substantially 100 dB with respect to the intensity.
In addition, as illustrated in NPL 2, a method for detecting a phase with respect to an optical path length of the terahertz wave is known, in which the method is performed by observing a change in the intensity of the detection light while causing the optical path length of the terahertz wave to transit by causing an idler beam along with the pump beam be incident on the nonlinear optical crystal on which the terahertz wave is incident.
On the other hand, a homodyne phase diversity detection system disclosed in PTL 1 is known as means for detecting a minute optical signal with high sensitivity in the field of information reproduction recorded on an optical disk although not in the field of measurement.